1. Field of the Invention
The instant invention is directed to a coating composition comprising a nitrocellulose, a copolymer of ethylene and an ethylenically unsaturated ester of a fatty acid and an organic solvent for the nitrocellulose and the copolymer. More preferably, the present invention is directed to a coating composition comprising nitrocellulose, a copolymer of ethylene and vinyl acetate and an organic solvent for the nitrocellulose and the ethylene-vinyl acetate copolymer.
2. Background of the Prior Art
The use of nitrocellulose in coating compositions has long been established in the art. Nitrocellulose lacquers provide protection and decoration on surfaces to which they are applied. Although nitrocellulose-based lacquers provide excellent coatings, these coating compositions are oftentimes found wanting when applied to polar surfaces. Particularly, polyolefin and metal surfaces, which those skilled in the art are aware are polar, are not satisfactorily coated with nitrocellulose coatings.
This problem has been addressed in the prior art. U.S. Pat. 3,321,420 to Unger advances a class of nitrocellulose-based coating compositions which include a copolymer of ethylene and an ethylenically unsaturated ester of a saturated fatty acid. Such coating compositions are recited in Unger to possess unusually good flexibility, scuff resistance, freedom from blocking, tensile strength, resistance to migration of ink colors to protective overcoats and good adhesion.
Although the Unger patent represents a significant advance in the art, especially in the coating of polar surfaces such as metals and polyolefin films, still certain problems associated with the use of these coatings are not addressed by that teaching. Of significant importance is the constituency of the organic solvent in which the polymers, nitrocellulose and a copolymer of ethylene and ethylenically unsaturated ester of a saturated fatty acid, are dissolved. Unger advises that the solvent system should include a hydrocarbon type solvent such as toluene and an organic polar solvent, for example, an alkanol such as butanol, an ester such as butyl acetate or a ketone such as methyl ethyl ketone.
The importance of environmental health and safety emphasizes the desirability of increasing the use of polar organic solvents, such as alkanols, and the concurrent decrease in the employment of aromatic hydrocarbons such as toluene, in coating compositions. Those aware of environmental problems appreciate that aromatic hydrocarbons are volatile and are suspected of detrimental health effects. Polar solvents, on the other hand, are less volatile and present far fewer health dangers. In the past, however, the solubility of a blend of nitrocellulose and a copolymer of ethylene and an unsaturated ester of a saturated fatty acid has prevented the concentration of polar solvents to be significantly increased. Thus, aromatic hydrocarbons, such as toluene, have predominated in nitrocellulose coating compositions.
The above remarks establish the need in the art for a new composition of nitrocellulose and a copolymer of ethylene and an ethylenically unsaturated ester of a saturated fatty acid having good coatability, especially on polar surfaces, in which the organic solvent system includes increased concentrations of polar solvents.